Power semiconductor devices are used as switching devices such as switching power supplies, inverters, etc. A high breakdown voltage and a low on-resistance are necessary in power semiconductor devices. The relationship between the breakdown voltage and the on-resistance is a trade-off relationship determined by the element material. In power semiconductor devices formed of Si, reductions of the on-resistance have reached the limits of the material. To further reduce the on-resistance, it is necessary to form the power semiconductor device of a material other than Si. As one such method, a HFET (Hetero-junction Field Effect Transistor) is formed of a nitride semiconductor of AlGaN and the like.
HFETs are field effect transistors in which a two-dimensional electron gas that forms at the heterojunction of AlGaN/GaN is used as the channel layer. Therefore, the mobility of the carriers is high. The two-dimensional electron gas is formed with a high concentration due to the piezoelectric polarization of the AlGaN layer occurring due to the lattice mismatch of the heterojunction. Further, the breakdown voltage of nitride semiconductors is high because the bandgaps are wide. As a result, a HFET formed of a nitride semiconductor (hereinbelow referred to as a GaN-HFET) has a better trade-off relationship between the breakdown voltage and the on-resistance, a lower on-resistance, and a higher breakdown voltage compared to a semiconductor device formed of Si.
However, the GaN-HFET is unfortunately a normally-on semiconductor device because the channel is continuously formed due to the piezoelectric polarization. Generally, it is necessary for a power semiconductor device used in a power apparatus such as a power supply, an inverter, etc., to be a normally-off semiconductor device to ensure the safety of the power apparatus. Therefore, a nitride semiconductor device that includes a GaN-HFET is artificially configured to be normally-off by the GaN-HFET having a cascode connection with a normally-off MOSFET formed of Si. Because the gate-drain capacitance is small in such a nitride semiconductor device that is artificially configured to be normally-off, noise due to the switching occurs easily. A normally-off nitride semiconductor device is desired in which the occurrence of noise due to the switching can be suppressed.